1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method and apparatus for improving the capacity of the reverse digital control channel by dynamically selecting the length of burst communications transmitted from a mobile station.
2. Description of Related Art
Every cellular telephone system is provided with a plurality of carrier frequencies for use in carrying communications between mobile stations and base stations. One-half of the carrier frequencies are normally allocated for carrying downlink communications from base stations to mobile stations, and the remaining one-half of the carrier frequencies are allocated for carrying uplink communications from mobile stations to base stations. The carrier frequencies are paired (one uplink and one downlink per pair) and distributed in accordance with well known cellular architecture techniques amongst the plurality of base stations.
Conventional cellular systems have historically been analog in nature with each carrier frequency comprising a physical channel (uplink or downlink) for carrying communications between base stations and mobile stations. In order to increase the capacity of the cellular system for handling communications, currently specified cellular systems are instead digital in nature implementing a time division multiple access (TDMA) communications protocol on each carrier frequency. A repeating TDMA frame (comprised of a plurality of time slots) is provided per uplink and downlink carrier frequency, with each time slot in the TDMA frame comprising a physical channel for carrying burst communications between base stations and mobile stations.
A great variety of information must be transmitted between base stations and mobile stations during cellular system operation. This information comprises both user data (voice traffic) and control signaling that is burst transmitted in certain ones of the time slots in accordance with the implemented TDMA protocol. Digital cellular systems include a number of different types of logical channels that are mapped into the physical channels (time slots) of the TDMA frame, with the logical channels identifying both the type of information that is sent and the order of transmission in accordance with the protocol. For example, user data (such as speech) is sent on the logical channels identified as traffic channels (TCHs). Certain ones of the physical channels in each TDMA frame, and in fact all of the channels in some TDMA frames, are reserved for the traffic channels. Control signaling is sent on the logical channels generically identified as control channels (CCHs). There are a number of different types of control channels. Certain ones of the physical channels in only certain ones of the TDMA frames are reserved for the control channels. Typically, the control channels are provided in the TDMA frames of only one pair of carrier frequencies per base station (cell).
Synchronization of the mobile stations within the cellular system to the repeating TDMA frames on both the uplink carrier frequencies and the downlink carrier frequencies assigned to each cell is vitally important to insuring proper system operation. Accordingly, it is well known for each base station to broadcast, on at least one of the control channels, a synchronization signal (burst) informing the mobile stations of the structure and timing of the TDMA frame. From the received TDMA frame structure and timing information, the location in time of each of the time slots (physical channels as well as logical channels) may be determined by the mobile stations and thereafter used to access the channels at the proper time to initiate and receive burst communications.
Because mobile stations are not located immediately adjacent to the base station, mobile station reception of the broadcast synchronization signal is delayed due to signal propagation delays and a corresponding forward time synchronization error is introduced at the mobile station in the TDMA frame. By this it is meant that the starting time of each time slot in the TDMA frame at the base station is slightly different from (i.e., earlier than) the starting time at the mobile stations. A reverse time synchronization error is also introduced due to signal propagation delays whenever the mobile station transmits a burst communication back to the base station. This mobile station burst communication, at what the mobile station incorrectly believes is the beginning of the selected time slot in the TDMA frame, is however received by the base station slightly after the mobile station TDMA frame time slot starting time, and significantly after the base station TDMA frame time slot starting time.
To account in most situations for the effects of the forward and reverse time synchronization errors, both base station and mobile station burst communications include an end guard period, and thus have a message portion with a conventional length that is less than the length of each time slot in the TDMA frame. With respect to mobile stations located close to the base station, the conventional length burst communication fits within the time slot in spite of any introduced forward and reverse time synchronization errors. In situations where the mobile station is located a much further away from the base station, however, the forward and reverse time synchronization errors become correspondingly quite large, and in fact may exceed the guard period and the time slot protections provided thereby. Thus, with large time synchronization errors, a conventional length burst communication transmitted from a distant mobile station during one given time slot may be received by the base station partially within both the given time slot and a subsequent time slot. Often this time slot interference results in the failure of the base station to receive the burst communication in either or both the time slots.
This time slot interference problem is of greatest concern during initial mobile station contact because the base station has not yet been given an opportunity to provide the mobile station with information directing the advancing of mobile station broadcasts in time and thus accounting for the introduced time synchronization error. One known solution is to have the mobile stations initially transmit a burst communication with a shortened (i.e., abbreviated) message length on the reverse digital control channel. A larger guard period is thus provided and the abbreviated length of the abbreviated burst communication is set to insure base station reception within the proper time slot almost regardless the length of the introduced time synchronization error.
The selection of abbreviated versus conventional length burst communications for mobile station use is typically made by the system operator on a cell by cell basis, with large cells being designated for only using abbreviated length burst communications at initial mobile station access, and smaller cells designated for conventional length burst communications. Control over mobile station operation in this regard is effectuated by transmitting a control message from each large cell base station designating the use of only abbreviated length bursts. This message is periodically sent in accordance with the logical channel assignment in the TDMA frame on the forward control channel. Changing of the abbreviated length designation has historically been made only by the system operator, with such changes being made infrequently, if at all, and only in response to system changes such as a change in the size of the cell.
While solving the concerns with time synchronization errors, abbreviating the message length of initial mobile station burst communications unfortunately adversely affects communications bandwidth (i.e., capacity) of the reverse control channel at initial access. A reduction in reverse control channel capacity of between thirty and forty percent is not unusual in situations requiring the use of abbreviated length bursts during initial mobile station contact with the base station. Another drawback to the use of abbreviated length bursts is that mobile station messages often must be divided into several parts and inefficiently transmitted to the base station during several different reverse control channel time slots. With such multiple abbreviated length bursts, there is a chance that one of the bursts may be only partly received by the base station, if even received at all, thus necessitating a retransmission of the message, again in multiple shortened bursts. Accordingly, it is preferable, even in large cells, that conventional length burst communications, rather than abbreviated length burst communications, be used whenever practical during initial mobile station access to the base station using the reverse digital control channel.